Oxidation resistant iron-chromium alloy



3,002,833 OXIDATION RESISTANT IRON- CHROMIUM ALLOY James A. McGurty, Cincinnati, and John F. Collins,

Hamilton, Ohio, assignors to General Electric (Zompany, a corporation of New York No Drawing. Filed Sept. 25, 1959, Ser. No. 842,519 1 Claim. (Cl. 75122.7)

This invention relates to high temperature, oxidation resistant alloys and, more particularly, to alloys of iron, chromium, yttrium and thorium.

For applications in air, the field of high temperature metallurgy is, for most practical purposes, limited to the utilization of oxidation resistant iron base and nickel base alloys, and the use of even these materials is limited to temperatures not in excess of 2000 F. The only other generally available materials which might be expected to have useful strength, oxidation resistance above 2000 F., and workability are chromium base or iron-chromium base alloys. While high temperature, high strength, structural, chromium base and iron-chromium base alloys do exist, the usefulness of such alloys has been limited by oxide film formation on the surface of such alloys wh ch does not afiord sufi'icient protection against further oxidation at high temperatures. The chromium oxide film which is formed during heating of the metal in air provides some useful protection at temperatures up to 2000 F. Above this temperature, oxidation is sufficiently rapid to discourage use of the metal at such elevated temperatures.

In copending application, Serial No. 842,517, filed September 25, 1959, in the names of James A. McGurty and John F. Collins, a ternary oxidation resistant ironchromium alloy is disclosed and claimed which consists of from 0.5 to 5.0 weight percent yttrium, from 20.0 to 95.0 weight percent chromium, and the balance being iron. Chromium base alloys with small amounts of yttrium are stable in air to temperatures well above 2000 F. These ternary alloys of iron, chromium and yttrium have good oxidation resistance at temperatures in excess of 2000 F.

Iron-chromium base alloys which would provide high strengflr, good oxidation resistance, and cladding protection for base alloys at temperatures in excess of 2000 F. are desirable. The invention of the present application provides such improved iron-chromium base alloys.

It is an object of our invention to provide an oxidation resistant alloy of iron, chromium, yttrium and thorium.

It is another object of our invention to provide an alloy which has superior oxidation resistance at temperatures in excess of 2000 F.

It is another object of our invention to provide an ironchromium base alloy with increased workability.

It is a further object of our invention to provide an iron-chromium base alloy with oxidation resistance at elevated temperatures which does not effect adversely the workability of the alloy.

In carrying out our invention in one form, from 0.1

atent base alloy having from 25 to 35 weight percent chromium and the balance of iron to provide a high temperature oxidation resistant alloy.

These and various other objects, features and advantages of the invention will be better understood from the following description.

We discovered unexpectedly that small additions of thorium and yttrium to iron-chromium base alloys produced alloys which had superior oxidation resistance in percent yttrium are combined with an iron-chromium air at temperatures in excess of 2000 F. The addition of from 0.1 to 1.0 weight percent of thorium, and from 0.5 to 5.0 weight percent yttrium to iron-chromium alloys having from 25 to 35 Weight percent chromium and the balance of iron provided these extraordinary efiects. While the above weight percents of thorium and yttrium improved oxidation resistance of this ternary alloy, from 0.5 to 3.0 weight percent yttrium appeared to be the optimum amount of yttrium addition to iron-chromium base alloys.

During the research which led to the discovery of these oxidation resistant iron-chromium alloys, a series of small vacuum melted iron-chromium-yttrium-thorium buttons were prepared. These alloy buttons, containing from 0.1 to 1.0 weight percent thorium, from 0.5 to 5.0 weight percent yttrium, from 25 to 35 weight percent chromium, and the balance of iron were heated in air at 2300 F. for hours. The specimens were then examined metallographically and weight changes measured. The data tabulated in Table I is representative of the excellent stability of the iron-chromium base alloys containing thorium and yttrium as compared to iron-chromium base alloys.

TABLE I Oxidation resistance of iron-chromium alloys [100 hours, air, 2300 F.]

We discovered that less than 5 weight percent iron in the iron-chromium-yttrium-thorium alloy was not significant. Furthermore, the addition of less than 20 weight percent of chromium effected adversely the oxidation resistance of this alloy.

While other modifications of this invention which may be employed within the scope of the invention have not been described, the invention is intended to include all such as may be embraced within the following claim.

What we claim as new and desire to secure by Letters Patent of the United States is:

An alloy of iron, chromium, yttrium and thorium which consists of from 0.1 to 1.0 weight percent thorium, from 0.5 to 5 weight percent yttrium, from 25-35 weight percent chromium, with the balance being iron.

References Cited in the file of this patent UNITED STATES PATENTS 2,190,486 Schafmeister Feb. 13, 1940 

